1. Field of the Invention
This invention relates to the use of certain urethane isocyanates containing (cyclo)aliphatically bound isocyanate groups as drying agents for moisture-containing auxiliaries and additives which are used in one-component coating compositions having (cyclo)aliphatic isocyanate prepolymers as binders and to a process for the production of a coating composition based on such NCO prepolymers.
2. Description of the Prior Art
Moisture-curing coating compositions containing organic polyisocyanates, more particularly NCO prepolymers of relatively high molecular weight, as binders have long been known (cf. for example H. Kittel, Lehrbuch der Lacke und Beschichtungen, 1973, Verlag W. A. Colomb, Vol. 1, Part 2, pages 573-576).
After application to a substrate, the coating compositions form films which, by taking up moisture from the atmosphere, crosslink to form ureas. The quality of the paint films and, above all, the stability of the ready-to-use coating composition in storage are critically determined by the use of dry auxiliaries and additives (such as pigments, fillers and/or solvents). Therefore, the additives have to be dried satisfactorily before production of the coating compositions. Highly reactive organic isocyanates have been successfully used as drying agents in this regard (DE-PS No. 1,245,590, DE-OS Nos. 2,539,728 and 3,204,128).
However, the use of highly reactive, low molecular weight isocyanate compounds, in particular tosyl isocyanate, is also attended by disadvantages including high vapor pressure and often extremely high reactivity which necessitates special precautionary measures. Most importantly, the isocyanates react with water to form insoluble secondary products, for example ureas, which crystallize out in paint compositions, which causes haziness and leads to film defects and poor levelling. This applies in particular to coating compositions containing isocyanate prepolymers (NCO prepolymers) having (cyclo)aliphatically bound isocyanate groups as binders because the secondary products formed adversely affect the weathering resistance of the paint films, thus destroying the major advantage of these binders over corresponding aromatic binders.
Accordingly, an object of the present invention is to provide new drying agents which may be used to prepare new coating compositions based on NCO prepolymers having (cyclo)aliphatically bound isocyanate groups which do not require the use of the highly reactive isocyanates mentioned above as drying agents, but which nevertheless possess excellent stability in storage in the absence of moisture and result in high-quality paint films.
This object may be achieved in accordance with the present invention as described in detail hereinafter.